Using a timeslip to help calculate TC slip, tire growth and slippage

A

Alky V6

Let's go racing, boyz!
Joined
Jul 29, 2001
I've been looking at ways to use a timeslip and a datalog with driveshaft speed info to determine torque converter slippage, tire growth percentage and tire slippage in the quarter mile.
Trying to figure this all out gets rather difficult so I thought I'd put it down here for review. Maybe this will help straighten out the thought process. Tire growth and tire slippage percentages can really muck things up.
 
The first step is to convert the mph of the car to driveshaft revolutions per minute. This will allow us to compare driveshaft rpm to engine rpm and get a torque converter slippage percentage.
Using my particular case as an example;

Tire diameter calculated by measuring the circumference is 28.6".
Rearend gear ratio is 3.73:1.
Speed sensor pulses per driveshaft revolution is 40.
Speed sensor pulses per mile is 105212.7.

If we take into consideration tire growth at the end of the quarter;

3.3% tire growth would equal a 29.54" tire diameter. The advertised diameter of the tire is 29.5".
Speed sensor pulses per mile with this new diameter is 102002.8.
 
Next let's simplify the number of pulses per mile by dividing that number by the number of pulses per driveshaft revolution.

For;
28.6" diameter tire that's 105212.7 / 40 = 2630.3175
29.54" dia tire that's 102002.8 / 40 = 2550.07
 
Now let's convert our mph numbers to miles per minute.

150 mph / 60 minutes = 2.5 miles per minute.
 
Now let's calculate torque converter slippage.

The following are example numbers.

Engine rpm: 7500.
Timeslip mph: 150.
Tire growth %: 0

150 mph / 60 = 2.5
2630 driveshaft rev/mile x 2.5 = 6575.
6575 / 7500 engine rpm = .8766
.8766 - 1 = .1233
12.3 percent slippage between the engine and the driveshaft speeds.
 
Now let's calculate torque converter slippage with tire growth.

Engine rpm: 7500.
Timeslip mph: 150.
Tire growth %: 3.3

150 mph / 60 = 2.5
2550.07 driveshaft rev/mile x 2.5 = 6375.175.
6375.175 / 7500 engine rpm = ..8500
.8500 - 1 = .15
15.0 percent slippage between the engine and the driveshaft speeds.

In this case, the difference when taking tire growth into consideration is a 2.7% difference.
 
What is happening when your datalog mph doesn't match your timeslip mph?
Two variables are at work here.
Tire slip percentage.
Tire growth percentage.

Tire growth with 0% of tire slippage would result in a timeslip mph that is greater than the mph recorded on the datalog.

145 datalog mph / 150 timeslip mph = .966.
.96 - 1 = .04.
4% tire growth.

The above is assuming there is no tire slippage percentage involved. With slicks involved, that is typically not the case.
 
What if the datalog mph is greater than the timeslip mph?
This indicates a percentage of tire slip is occurring, but, along with an expected percentage of tire growth.
In this case, simply doing the math to get a percentage difference between the two mph figures will not show only tire slip percentage, since there will be a certain amount of tire growth included in the answer.

If you have a good percentage number for the tire growth at that mph, you can back out that percentage in your answer to have a clearer view of what the actual tire slip percentage is.
 
Establishing a baseline tire growth percentage.

I was wondering, how would I figure out what my tire growth percentage is for my situation, since tire slippage could easily muddy up any attempt to figure out tire growth?
You can contact the manufacturer of the tire and they should be able to give you an idea of what the tire growth percentage is for the tire, or you can figure it out for yourself. Figuring it out for yourself would be more accurate.

There are a few variables that can mess up your calculation of your tire growth.
Incorrect number being used for the tire diameter. MEASURE IT. Do not go by what is displayed on the side of the tire.
Incorrect final drive gear ratio.
Incorrect numbers being used for speed sensor pulses per driveshaft revolution.
Incorrect calculated pulses per mile.
Tire slippage present.

Assuming all the basic variables are accurate, how do we eliminate the tire slippage variable?

During a run down the quarter, accelerate up to the 1,000 ft mark and roll back on the throttle so that you pass through the timing area at a steady cruise. Not accelerating, and not deaccelerating. This will minimize any tire slip and should give you a very accurate mph number to use in your tire growth calculation.
Once you've established a good tire growth percentage number, you can simply back that number out of any future calculations where you are trying to determine tire slippage.
Having a good solid tire growth percentage will also make any torque converter slippage numbers much more accurate.
 
Next.
How many methods are there for coming up with a tire diameter number?
Which method is the most accurate?

Static rolling radius
Dynamic rolling radius
Tire circumference
Tire rolling circumference. I like this one.

Measuring the dynamic rolling radius is a little out of my league. The other methods are easy enough.
 
Static rolling radius. With the car sitting on the ground loaded as normal and with the tire air pressures at the expected level to be used, measure from the axle centerline directly down to the ground. Double that figure to come up with the diameter.
To calculate the circumference, multiply the diameter by pi (3.14159).
 
Tire rolling circumference.
There are two ways to come up with an answer for this one.
One way is to calculate the rolling circumference is by using the dynamic rolling radius. We don't have a laboratory to come up with that figure so we'll skip this method.
The other method is to roll the car on the ground exactly one tire revolution and measure the distance traveled on the ground. The tires should be aired up to the expected level to be used, and the car should be normally loaded.
Once you have this rolling circumference measurement, divide the number by pi (3.14159) to come up with a tire diameter figure.
 
I went ahead and did the rolling circumference exercise on my car. I think the last time I measured my tire I simply measured the circumference of the tire with the car up in the air on a rack. Let's see what the difference is between the two methods. The tire is a little more worn than before.

I marked a point on the sidewall and on the floor at as close to 6 O'clock as I could guess. I also marked the sidewall at 12 O'clock with a mark on the fenderwell lip opposite the mark on the sidewall. This will be my one turn of the wheel indicator.
I pushed the car straight ahead until the mark at 12 O'clock again lined up with the mark on the fenderwell lip. I thin marked a point on the floor next to the mark on the sidewall that was previously put at the 6 O'clock position.
Using a tape measurer, I measured between the two marks on the ground and came up with 92.1875. 9 and 3/16". This works out to 28.5" diameter.
My previous measurement was 28.6". Pretty close.

My new pulses per mile is 105581.8
The new driveshaft revs/mile is 2639.545.
 
Next weekend I'll be taking the car out to the 1/8 mile to get an accurate tire growth number. After I have that number in my clutches, anything will be possible. Mwahahaaaaaaa. :eek: Ooo. Sorry. Didn't mean to scare you.
 
Nice calculators, but I don't see that they are taking into account tire growth, or tire slippage.
I'm going through some chassis tuning and want to be able to track tire slippage. Inorder to do that accurately, you need to first know what the tire growth percentage is so that it can be backed out of the calculation.
 
What if the mph on the timeslip closely matches the mph shown in the datalog, but the car feels a bit loose on the top end?

First off, slicks are known for having varying degrees of tire growth depending on the particular tire. The range is generally 3 to 30%! You will have tire growth with a slick.

From what I understand, drag radials are not known for growing. I could easily be wrong about this, because I don't have any experience with drag radials. In the real world, I would imagine there is some amount of growth. Most likely, a small number due to the type of construction.

Let's say the mph is the same on both the timeslip and the datalog, and you're running slicks. Let's also say the mph is 160. With slicks we know there has got to be some percentage of tire growth going on. Especially at 160 mph.
From the calcs above, we've already determined that tire growth will lower the datalog mph compared to the timeslip mph, so if the mphs are matching, then the only explanation is that there is an offsetting amount of tire slippage.
Being able to recognize and track that percentage of tire slip can help with chassis changes.
 
Holy smokes donny my head is going to explode.......good info thou thanx......

donny you coming to the car show on saturday 4-30-11 beacuse im making the best mexican tacos just for you.....hope to see there.
 
You must log in or register to reply here.

Similar threads

B
10/14/2008 Gateway-SCSS Street Car Shootout RESULTS!
Replies
1
Views
1K
Tony Shoaff
T
Top